What are these black wall objects in Half Life 2?

I'm replaying Half-Life 2 and I'm puzzled about these objects:

What are these? What is their function? They sometimes move and crash into walls. It seems they are some kind of containment measure from the Combine's part but I did not find any explanation so far.

Solution 1:

I would try to explain it myself, but in truth when I came across the walls I never really thought twice of them. I did however Google "Half Life 2 walls" with a successful result.

http://half-life.wikia.com/wiki/Combine_Smart_Barrier

This should explain what they are far better than anything I could come up with.

The Combine Smart Barrier, also known as the Combine Mobile Wall, is a large mechanized wall that uses powerful drive systems in tandem with heavy metal blocks to slowly expand outward and destroy existing structures in the way.